Piezoactuators are known in which piezoceramic layers are arranged one on top of another, in a stack. Internal electrodes are arranged between the ceramic layers. When an external voltage is applied to each of two opposingly arranged electrode layers, the resulting electrical field is capable of causing a slight deflection in the ceramic layer due to the piezoelectric effect. The effect of these deflections of the ceramic layers placed on top of each other is cumulative in the lengthwise direction.
In order to be able to assure external contacting of the internal electrodes in piezoactuators by simple means, a passive zone is usually provided at the edge of the piezoactuator, in which only internal electrodes assigned to the same electrical pole lie on top of each other. At this point, the internal electrodes assigned to the opposite electrical pole do not extend quite as far as the edge of the actuator, they are limited to an area inside the actuator. Unlike the active zone, i.e. the zone where electrode layers of opposite polarity are arranged on top of each other, almost no extension of the piezolectrical layers takes place in the passive zone when an electrical voltage is applied, which causes tensile stress of the passive zone in the edge area of the passive zone, caused by the active zone. The more piezoelectric layers are arranged on top of each other, and the greater the applied electrical voltage is, the greater are the tensile stresses that occur at the edge of the passive zone.
These tensile stresses may cause cracking in the edge zone of the piezoactuators, either while the piezoactuator is operating, or even earlier during polarizing, i.e. in the first process step for operating the piezoactuator, in which an electrical polarization is created in the piezolectric layers. When the piezoactuator is operated continuously, these cracks may extend into the interior of the actuator and may break through one or more internal electrodes in the active zone. This may then cause dielectric breach at these breakage points, with slag formation and possibly even an internal short circuit in the component. Steps are taken to avoid such cracks in the internal electrodes.
In order to avoid such cracks, it is known from Patent No. 199 28 178 A1 to divide the piezoactuator into several partial actuators, wherein the partial actuators are stacked on top of each other following their manufacture and affixed by bonding. In this way, tensile loads at the inner edge of the passive zone cannot accumulate over the entire height of the piezoactuator. Instead only the tensile stresses that are created within the partial actuator are significant. However, these are relatively small, since the partial actuator only includes a very small number of superposed piezoelectric layers.
The drawback of the method described in that document is that the geometrical precision when placing individual actuators on top of each other is highly inadequate. In addition, stacking several partial actuators entails an additional manufacturing step, which leads to higher costs.
In order to prevent the formation of cracks in the internal electrodes of piezoactuators, it is also known from German Patent No. DE 198 02 302 to distribute the passive zone over the entire periphery of the piezoactuator with a construction method in which ach component is rotated through 90°. However, this construction has the disadvantage that the external contacting is complicated thereby, since the piezoactuator must be contacted from all four external sides.
From German Patent No. DE 100 16 428 A1, it is further known to balance the stress at the respective point of the passive zone via additionally applied insulating layers in the passive zone. The disadvantage of this construction is that it manufacturing such a piezoactuator is more expensive.